1. Field of the Invention
The present invention relates to a solid-state image pickup device and an optical instrument, particularly to the optical instrument in which shading, in which a peripheral portion of an image becomes dark, and image distortion due to aberration are not generated in a wide-angle mode, a telephoto mode, and a close-in mode, and the solid-state image pickup device used for the optical instrument.
2. Description of the Related Art
Recently, an electronic endoscope or a digital camera using a charge coupled device (CCD) for forming the image is widely used.
Usually, a micro lens is formed on a surface of CCD, and sensitivity is improved by effectively collecting light incident from an optical system onto the surface of CCD with the micro lens.
In the electronic endoscope, the miniaturization of the instrument is achieved by extremely decreasing a focal distance of the optical system such as the lens as short as 2 to 3 mm.
When CCD has a plane shape, a light beam incident from the optical system is obliquely incident to the micro lens in an end portion of a light receiving surface of CCD. At this point, because the micro lens is formed in a spherical shape of bilateral symmetry, the light obliquely incident to the micro lens does not reach to the light receiving surface of CCD. This allows generation of the shading in which the end portion of an image plane becomes dark.
The aberration in which distortion is generated in the image by difference in focal distance from the optical system between the central portion of the light receiving surface of CCD and the end portion of the light receiving surface becomes troublesome in the digital camera, so that the number of lenses is increased to reduce the aberration (curvature of field). Further, a decrease in ambient light quantity is reduced by the so-called lens shift in which the center of the micro lens is offset from the center of the light receiving portion of CCD.
However, it is necessary to decrease the number of lenses. In the lens shift, when an image pickup lens is different, since the amount of shift of the micro lens is also different, it is necessary to produce the new CCD. Further, there is a problem that the correction of the aberration of the image pickup lens by the lens shift is incomplete.
Therefore, in the electronic endoscope and the digital camera, that CCD is curved toward the optical system has been investigated (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 6-133229 and No. 2001-156278).
Alternatively, there has also been proposed a configuration provided with curvature adjusting means which adjusts a curvature by which a surface of a solid-state image pickup chip is curved (for example, see JP-A No. 10-108078).
However, in the case where a curvature of CCD is fixed at constant in the digital camera, as long as many lenses are not used in the optical system, there is the problem that the distortion of the image is easy to occur, when wide angle shooting is performed while the curvature of CCD is set to the telephoto mode.